Brønsted/Lewis Acid Synergy in Dealuminated HY Zeolite: A Combined Solid-State NMR and Theoretical Calculation Study

Abstract

The Brønsted/Lewis acid synergy in dealuminated HY zeolite has been studied using solid-state NMR and density function theory (DFT) calculation. The 1H double quantum magic-angle spinning (DQ-MAS) NMR results have revealed, for the first time, the detailed spatial proximities of Lewis and Brønsted acid sites. The results from 13C NMR of adsorbed acetone as well as DFT calculation demonstrated that the Brønsted/Lewis acid synergy considerably enhanced the Brønsted acid strength of dealuminated HY zeolite. Two types of Brønsted acid sites (with enhanced acidity) in close proximity to extra-framework aluminum (EFAL) species were identified in the dealuminated HY zeolite. The NMR and DFT calculation results further revealed the detailed structures of EFAL species and the mechanism of Brønsted/Lewis acid synergy. Extra-framework Al(OH)3 and Al(OH)2+ species in the supercage cage and Al(OH)2+ species in the sodalite cage are the preferred Lewis acid sites. Moreover, it is the coordination of the EFAL species to the oxygen atom nearest the framework aluminum that leads to the enhanced acidity of dealuminated HY zeolite though there is no direct interaction (such as the hydrogen-bonding) between the EFAL species and the Brønsted acid sites. All these findings are expected to be important in understanding the roles of Lewis acid and its synergy with the Brønsted acid in numerous zeolite-mediated hydrocarbon reactions.